(a) Field of the Invention
This invention relates to an improved process for vapor degreasing using a chlorinated C.sub.2 hydrocarbon, or more particularly a mixture of perchloroethylene and water, as a solvent.
Vapor degreasing is a highly effective process for physically removing solvent soluble soils and other entrapped soils from metal, glass, plastic, coated items and other essentially non-porous articles that are not affected by the solvent. In vapor degreasing the selected liquid solvent is evaporated from a reservoir, the vapors are condensed on the soiled articles whereby the condensate washes off the soil and then returns to the reservoir for reuse.
(b) Description of the Prior Art
Solvents heretofore used in vapor degreasing include 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113), methylene chloride, 1,1,1-trichloroethane, perchloroethylene and trichloroethylene, among others. Many blends such as for example, CFC-113 plus methylene chloride or 1,1,1-trichloroethane plus propanol are also used in the metal and electronics cleaning industries.
Solvents are chosen for specific applications based on the physical properties of the individual solvent or blends of solvents. Important properties include boiling point which dictates the temperature to which a part being cleaned will be heated, toxicity which limits acceptable worker exposure rates, and flammability which limits the range of solvent blends which can be safely used. Evolving regulations of solvent emissions due to contributions to ozone depletion and/or greenhouse effect are increasingly affecting the choice cf solvent and methods used in metal and electronics cleaning.
Solvent properties such as boiling point, flammability, solvent power and even solvent toxicity can be adjusted by mixing several solvents together. However, the use of simple blends can be unacceptable in vapor degreasing due to fractionation of the blends to an undesirable degree. The fractionation of the solvent blend during distillation can make the solvent mixture nearly impossible to recover for reuse at the original composition.
A partial solution to the fractionation problem has been to use certain azeotropic mixtures of organic solvents having constant composition characteristics and constant boiling points. The vapor degreasing and vapor defluxing systems act as a distillation still. Unless the solvent is a pure material having a constant boiling point, or is an azeotrope or azeotrope-like mixture, fractionation will occur and undesirable solvent distribution will occur which can affect the efficacy and safety of the cleaning operation.
Azeotropes are either maximum- or minimum-boiling in nature, having a boiling point above or below that of any of the components in the mixture. Minimum-boiling azeotropes can be beneficial in metal and electronic cleaning operations, as the lower boiling point results in a cooler part temperature, rendering parts handling much safer to operating personnel.
As is well known, azeotropes can be formed with combinations of polar and nonpolar solvents. Use of a mixture of 1,1,1,-trichloroethane and n-propanol is a common example of such an azeotropic system. This formulation has been found useful in the commercial cleaning of electronic components and ionic contaminants from metal or other surfaces. However, 1,1,1,-trichloroethane becomes relatively easily decomposed in the presence of water, giving rise to acidic, corrosive components in the system, and consequently the field of use for this solvent system has had its limitations.
Vapor degreasing is of course broadly old. A general description of the process, its various: modifications, its applications, and the equipment used can be found, for instance, in Manual on Vapor Degreasing, Third Edition, ASTM Manual Series: MLN 2, Revision of Special Technical Publications (STP) 310 A, June 1989, the full text of which is hereby incorporated by reference in this patent specification.
As described in this ASTM Manual in FIG. 1b, in its simplest form a solvent vapor degreasing machine is a tank with a heat source, e.g., a steam coil, to boil the solvent in a boiling sump or reservoir located in a bottom portion of the tank and a cool annular surface at an intermediate or upper section of the periphery of the tank, which causes the solvent vapors to condense on the wall and run down and defines a solvent vapor level in the tank above which the solvent vapors do not diffuse to any substantial extent, with an air space above that level. The annular cool surface can be formed, for instance, by means of a water jacket located at a suitable height around the tank and/or condensing coils located inside the tank. At the same time, as the soiled parts or articles are at approximately ambient temperature when they are introduced into the degreasing tank and suspended in the air-free zone of solvent vapor, the hot vapors condense onto the cool parts, dissolving greases and other soluble contaminants and providing a continuous rinse in clean solvent. As the condensed solvent drains from the parts, it carries off the soils and returns them to the boiling sump proper or to an adjacent condensate reservoir. In the operation of the present invention, all the condensed perc and water which thus return to the liquid reservoir from above as a result of condensation of the azeotropic vapor mixture on the parts being cleaned or on the internal cooling coils or on the cooled tank wall will normally overflow from the reservoir to the sump for further evaporation once the reservoir becomes filled with the high-density perc. However, a water layer of adjustable depth can be maintained on top of the perc in the condensate reservoir by suitable modification of the equipment and its operation. More particularly, the depth of such water layer consequently covering the condensed perc layer in the reservoir can be readily adjusted as may be desired, taking advantage of the much greater density of perc versus water.
For instance, as illustrated in FIG. 2 of the drawing, a perc transfer line 40 which is fitted with a height-adjustable overflow line 42 can be added connecting the reservoir to the sump. Increasing the height of the overflow will raise the level of perc in the reservoir. At a sufficiently high level all perc and water will overflow a weir placed at the upper edge of the reservoir and no perc will flow &o the sump through the transfer line, which will result in a minimal level of water remaining to cover the perc in the reservoir. By lowering the overflow line the perc layer height in the codensate reservoir will drop with a substantial water layer forming above the perc up to the weir overflow. During operation at such a setting, all subsequent condensed perc will flow through the perc transfer line back to the boiling sump while the condensed water phase will overflow the weir of the reservoir back to the boiling sump. Adjusting the overflow line height will also result in a higher or lower depth of water being maintained in the boiling sump.
Vapor treatment in degreasing is often augmented by mechanical action, for instance, by immersion of the soiled parts in a section of the tank containing liquid perc solvent before the parts are raised into the vapor zone for vapor degreasing, or by additionally spraying the parts with liquid perc or water while they are in the vapor degreasing zone. Ultrasonic agitation of the solvent may also be employed while the soiled parts are immersed therein.
The parts are usually held in the vapor zone for rinsing by the condensing vapors until the parts reach vapor temperature, at which time condensation stops. The parts dry quickly within the machine as they are withdrawn from the vapor into the air space. A hood may be provided above the tank further to minimize diffusion of solvent vapors into the atmosphere.
In a conveyorized cross rod degreaser, such as that described in FIG. 5 on page 12 of the previously mentioned ASTM Manual on Vapor Degreasing, baskets containing the work can be automatically transferred from a roller conveyor, without the necessity of transferring the parts before and after the cleaning process. Large parts can be handled or a monorail from which the parts are suspended. If the monorail is suitably contoured as shown in FIG. 2 on page 4 of the ASTM Manual, the suspended parts can thus pass in a continuous sequence from the outside through the vapor degreasing zone, where cleaning may be augmented by use of a liquid solvent spray, and or out of the chamber.